do not call g_thread_init() for glib >= 2.31

glib >= 2.31 always enables thread support and g_thread_supported()
is #defined to 1, there's no need to call g_thread_init() anymore,
and it definitely does not need to report error which never happens.
Keep code for old < 2.31 glibc anyway for now, just #ifdef it
differently.
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Cc: qemu-trivial@nongnu.org